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Question: How is calcium distributed in the body and in the serum?


A 70-kg man has approximately 1000 gm of calcium in his body. Of this amount, bone contains 99%, whereas the ECF and ICF contain only 1%. Furthermore, only about 1% of skeletal calcium is freely exchangeable with ECF calcium. The routine measurement for serum calcium (normal = 9-10 mg/mL = 4.5-5.0 mEq/L = 2.25-2.5 mM/L) measures total calcium. Approximately 40% is protein-bound, 5-10% is complexed to other substances (e.g., phosphate, sulfate), and 50% is ionized.

Reference: Rose BD, Post T (eds): Clinical Physiology of Acid-Base and Electrolyte Disorders, 5th ed. New York, McGraw-Hill, 2001.

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